My son's preschool teacher was opposed to early kindergarten entrance as well, even though DS was more than ready. I suspect that on some level she saw this as a rejection of the preschool and was simply acting defensively, rather than in the best interest of the student.
In our case, having an early skip built in with early enrollment made for less disruption later on, when DS needed to be skipped again. His classmates see him as having skipped only one grade, when actually he's two years younger.
